In a typical commercial mill for the treatment of cellulosic fiber material to form paper pulp, the fiber material would typically be digested to a Kappa number of about 35, and the fiber material would be bleached to reduce the Kappa number to any desired value. For instance oxygen bleaching could take place to reduce the Kappa number from 35 to 15, and then subsequently chlorine bleaching could be used to further reduce the Kappa number. Conventionally, bleaching takes place either with high equipment and energy penalties and, or with a relatively low pulp yield per ton of fiber material treated.
Existing equipment for effecting oxygen bleaching presently takes the form of one of a variety of oxygen reactors. A first type of oxygen reactor such as shown in U.S. Pat. No. 3,660,225, utilizes equipment to increase the consistency of the digested pulp (normally about 8 to 15%) to about 20 to 30% before treatment, and then the pulp is fed to the reactor for treatment. A second type of oxygen reactor is shown in U.S. Pat. No. 3,832,276, which employs apparatus for thinning the pulp from the normal digester consistency to about 2 to 5% consistency, and then feeding the pulp to the oxygen reactor. A third approach, such as shown in U.S. Pat. No. 3,963,561, employs a different type of oxygen reactor and can treat the pulp at the digester consistency of about 8 to 15%. While all such apparatus is effective for achieving the desired results, the capital investment is normally quite high, and the energy penalty associated with treatment--especially where separate dilution and thickening steps must be employed prior and/or subsequent to bleaching--is normally quite high, and the Kappa number often cannot be reduced low enough to avoid some chlorine bleaching.
In conventional systems, chlorine is used to reduce the Kappa number from 35 to about 6, with subsequent steps taken to whatever level of brightness is desired. Chemical cost is greater than for oxygen bleaching, however, and the pollution load increases significantly since the spent chemical solution cannot be returned to the system.
According to the present invention, a method and apparatus are provided which can effect oxygen bleaching at reduced capital cost and at reduced energy penalty compared to existing commercial insulations, with a reduced chemical use and pollution load compared to chlorine bleaching, and with increased yield compared to existing commercial installations.
According to one aspect of the method according to the present invention, digested pulp is treated by adding oxygen and sodium hydroxide solution thereto, to effect oxygen bleaching, reaction products being formed during oxygen bleaching, and removing the reaction products substantially immediately after formation thereof, and during oxygen bleaching, from the digested pulp. The adding and removing steps are consecutively repeated until pulp bleached to desired Kappa numbers produced. Preferably the pulp is at a consistency of about 8 to 15%, and the removal step is accomplished by thickening the pulp to about its original consistency of about 8 to 15%. In this way, digesting can be terminated at a Kappa number of about 70 (instead of 35 as conventionally) with oxygen bleaching effecting the reduction of the Kappa number from 70 to 15 or even below 15.
According to another aspect of the method according to the present invention, cellulosic fiber material is treated by digesting the fiber material to produce digested pulp having a first consistency of about 8 to 15%, and then the pulp is oxygen bleached by mixing the pulp with O.sub.2, NaOH solution, and water, thereby forming an integral mixture of pulp with O.sub.2 at a second consistency less than said first consistency, thickening the integral mixture to return it to a consistency generally the same as the first consistency, and subsequently mixing and thickening until a desired level of oxygen bleaching is achieved. The oxygen bleached pulp is subsequently washed. The temperature and pressure conditions at which the bleaching is accomplished may be continuous digester conditions--a temperature over 100.degree. C. and superatmospheric pressure.
The method, according to the present invention, may be accomplished with minimum capital expenditure by utilizing the apparatus according to the invention, the apparatus including a plurality of mixing means disposed in series in a line (i.e. digester discharging line) and including first and last mixing means, for mixing digested pulp with O.sub.2, NaOH solution, and water; a plurality of thickening means disposed in the line in series and including a first and last thickening means, each thickening means being disposed in the discharge from a mixing means, and means for adding O.sub.2, NaOH solution, and water to the mixing means. Means are also provided for washing the pulp discharge from the last thickening means of the series. Each thickening means preferably includes an in-line thickner capable of thickening pulp at superatmospheric pressures and at a temperature over 100.degree. C. such as the thickener shown in co-pending application Ser. No. 676,660, filed Apr. 13, 1976, now U.S. Pat. No. 4,041,560; and Ser. No. 676,659, filed Apr. 13, 1976, now U.S. Pat. No. 4,029,579. Means are provided for recycling back to a previous mixing means in the series, liquid withdrawn from a thickening means to provide the water for mixing in the previous mixing means, and the mixing water for the last mixing means in the series is provided from the washing means, while the separated liquid from the first thickening means in the series is circulated back to effect washing in a continuous digester. The mixing means may comprise fibrilizing means for effecting at least partial fibrilizing of the pulp, and means may be provided for separating shives from the pulp discharge from the fibrilizing mixing means and for returning the shives to the fibrilizing mixing means while passing the pulp to a subsequent thickening means.
It is the primary object of the present invention to provide a method and apparatus for treating cellulosic fiber material to obtain a high pulp yield with minimum chemical use and energy penalty, and minimum capital expenditure, including oxygen bleaching of the pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.